1. Field of the Invention
The invention relates to a direct current (dc) power supplying system, more particularly to a voltage balancing device that permits voltage outputs of two or more power supplying units of the power supplying system to become balanced so as to extend the service life of the power supplying units.
2. Description of the Related Art
Conventional electronic devices, such as mobile telephones and notebook computers, require a rechargeable battery pack which can supply electric power to the electronic devices. The rechargeable battery pack generally includes at least two rechargeable batteries, such as lithium battery, connected in series. Each of the batteries has different internal leakage current and generates a different voltage output after a period of time so as to reduce the capacity of the rechargeable battery pack. After a long period of use, the difference between the output voltages of the batteries increases, and the output voltages of the batteries can not become balanced. The result is that the batteries can not be used even after being fully charged. For example, assuming that two lithium batteries A, B are connected in series, and that output voltages of the lithium batteries A, B are 3 volts and 2.5 volts, respectively, after a period of time due to the generation of leakage current. Usually, a lithium battery is fully charged at 4.2 volts and is fully discharged at 2.5 volts. When the lithium batteries A, B are charged until the output voltage of the lithium battery A is 4.2 volts, the output voltage of the lithium battery B is at most 4 volts. When the discrepancy between the output voltages of the lithium batteries A, B becomes larger, repeated charging of the batteries A, B will eventually result in failure of the same.